This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Extremely high x-ray densities in a semiconductor target will be obtained with a focused pink beam. The resultant many-body response will be probed primarily by measuring optical absorption near the band edge as well as optical reflectivity when possible. A pink beam will deliver up to 105 x-ray photons per femtosecond. When focused at the selected beamline on a GaAs target the instantaneous density of absorption sites is ~1012 cm-3. While this is low compared to traditional semiconductor doping densities this concentration of deep core holes will generate a cascade of many body responses: band-narrowing photoelectrons Auger electrons x-ray fluorescence thousands of electron-hole pairs per x-ray photon electron-plasmon interactions electron-phonon interactions etc. Band-edge absorption measurements will be made with picosecond laser pulses in this x-ray pump/optical probe study. This is perhaps the first synchrotron study to explore semiconductor many body responses to hard x-rays part of the vanguard of increasing interest in using high x-ray energy densities to explore condensed matter physics.